Magnetic methods are known to be “nonunique” in their interpretation, but certain generalities are usually true. For example, it is fair to say that most highamplitude magnetic anomalies indicate large contrasts in susceptibility, which in turn indicate large variations in intrabasement rock type. “High amplitude” means anomalies on the order of 150 or 200 nT (gamma) or more. The figure here shows the difference between a simple susceptibility contrast (about a 300-nT anomaly) and a large structure (1219-m, or 4000-ft, fault, producing a 25-nT anomaly). If such features are superimposed on each other, it can be very difficult to separate the effects of structures and susceptibility contrasts. Inasmuch as major intrabasement contacts are often weak zones that may be rejuvenated by later tectonism, this separation is important, even though it may be difficult.